Nuclear density gauges are widely used in industry to monitor variations in flowing material. Essentially, these gauges consist of a source of radiation which is directed across the flow path of the material toward a detector. The source and detector are usually positioned exteriorly of the pipe, conduit, duct, hopper, or other device through which the material is flowing and as the radiation (e.g. gamma radiation) passes through the flow, a portion of it is absorbed by the material in the flow. The amount of radiation absorbed by the material in a unit volume of the flow is directly proportional to the mass in that unit volume. The higher the mass, the more radiation is absorbed and the less radiation arrives at the detector. Since the amount of absorbed radiation is directly proportional to the mass of the unit volume, the radiation measured at the detector is, therefore, inversely proportional to the mass of the unit volume. The density of a material is its mass per unit volume and since the intensity of the radiation arriving at the detector is inversely proportional to it, fluctuations in the density of the flowing material can be easily and quickly determined by monitoring the fluctuations in the intensity of the radiation arriving at the detector. Depending upon the type of radiation (e.g. alpha, beta, gamma) and the material in the flow, the incoming radiation may be absorbed, scattered, or absorbed and scattered by the material; however, in all of these circumstances, the intensity fluctuations in the transmitted radiation arriving at the detector are still inversely proportional to the mass per unit volume and can be used to accurately monitor density fluctuations in the flowing material.
Nuclear density gauges are safe, accurate, and reliable. In many instances, they are preferred over other measuring techniques because they do not interfere with the flow and can give a continuous reading of the condition of the flow. Further, unlike measuring techniques such as sampling, the flow being monitored by the nuclear density gauge need not be removed from the pipe, hopper, or other device. This is especially important in situations where the material is difficult to handle, messy, or cannot be removed from the pipe, hopper, or other device without affecting the very property or properties being measured.
The source of radiation in a nuclear density gauge is generally a radioactive material emitting high energy photons. Gamma radiation is widely used in nuclear density gauges because it is relatively unaffected by the chemical composition of the material and the amount of gamma radiation absorbed by the material is directly proportional to its mass. Cesium 137, strontium 90, and like elements whose radioactive half-life is sufficiently long for practical use are typical sources of gamma radiation that are used in nuclear density gauges. Other commercially available density gauges include those using lower electromagnetic radiation than that emitted by radioactive materials and those using sound waves in place of any radiation.
Nuclear density gauges are commonly used as on-off switches and warning devices that react to the absence or presence of a single, predetermined condition in material flowing through a duct, pipe, conduit, hopper, or the like. They are also commonly used to monitor fluctuations in the overall density of a flow and, in certain circumstances, they can further be used to accurately monitor fluctuations in the amount of a single component in the flow. For a nuclear density gauge to be able to monitor fluctuations in the amount of a single component in a flow, the flow must have only two components and one of the components must have a fixed specific gravity. In a two-component flow where one component has a fixed specific gravity (e.g. water), variations in the amount of the second component (e.g. particulate solids) will cause a predictable change in the radiation received by the detector. If a third component is present in the flow, it will adversely affect the accuracy of any readings because it represents a second, uncontrolled variable. The problems presented by a third component are particularly acute if its specific gravity is drastically different from that of the other components. For example, in a slurry of water (specific gravity 1.00) and particulate solids (specific gravity 2.7), the presence of air (specific gravity 0.001) in any significant amount as bubbles, an air column, an air core, or the like will make it impossible to use a nuclear density gauge to monitor fluctuations in the density of the particulate solids in the flow. In such three-component flows, there is no way to accurately determine the relative portion of the radiation arriving at the detector that is due to the presence of air as opposed to the presence of particulate solids.
U.S. Patents and Defensive Publications filed in the U.S. Patent Office that illustrate the use of nuclear gauges as on-off type switches or warning type devices reacting to a single variable or condition in the flow are: U.S. Pat. No. 3,796,692 to Foltz et al. issued on Mar. 12, 1974, U.S. Pat. No. 3,545,735 to Wolf et al. issued on Dec. 8, 1970, and Defensive Publication T913,010 by Arnold et al. corresponding to U.S. Patent application Ser. No. 341,475 filed on Mar. 15, 1973. The patent to Foltz involves a polymerization process in which a preferred level 30 of molten polymer in reactor 18 is first determined. Radiation from source 32 is then passed through the reactor 18 at the preferred level 30 along a horizontal path toward the radiation sensor 34. The absence or presence of molten polymer at level 30 is then sensed and Foltz's controls operated accordingly to maintain the molten polymer at the level 30. In a substantially corresponding manner, Wolf uses vertically spaced radioactive indicators at 14 and 15 to activate the closure valve 3 in response to the sensing of a maximum and minimum level of dust in the container 2 by the indicators. In the Defensive Publication of Arnold, nuclear density gauges are set up along a pipeline to react to the presence of sand in the normal well fluid. As explained by Arnold, the nuclear density gauges "can be set to energize an alarm, immediately or after a preset time delay, when the detected specific gravity of the well fluid varies beyond a preset minimum and/or maximum."
Numerous U.S. patents illustrate the use of nuclear density gauges to monitor density fluctuations in material flowing through a conduit, hopper, or the like. Among these are: U.S. Pat. No. 3,582,647 to Figuet et al. issued on June 1, 1971, U.S. Pat. No. 3,529,153 to Zimmerman et al. issued on Sept. 15, 1970, U.S. Pat. No. 3,281,594 to Garrison issued on Oct. 25, 1966, U.S. Pat. No. 3,128,786 to Badgett issued on Apr. 14, 1974, U.S. Pat. No. 3,106,933 to Kloppel issued on Oct. 15, 1963, U.S. Pat. No. 3,577,158 to Hahn issued on May 4, 1971 and U.S. Pat. No. 3,208,592 to Smith issued on Sept. 28, 1965. Figuet uses his nuclear density gauge to monitor density fluctuations in particulate solids flowing through a hopper 1. He also employs his nuclear density gauge as an on-off switch which responds to the absence or presence of material at a predetermined level in the hopper 1. Zimmerman's nuclear density gauge 1 is used to monitor density fluctuations in material flowing through his pipe 2. Garrison and Badgett each illustrate a nuclear density gauge in use monitoring a flow of sludge passing through a pipe. Kloppel's nuclear density gauge monitors density fluctuations in material at the bottom of a sedimentation tank 23. The material being monitored by Kloppel has a liquid phase and a solid phase. In the patent to Hahn, a nuclear density gauge is used to monitor density fluctuations in a flow passing through the conduit 12. Smith employs a plurality of nuclear density gauges 22, 23, and 25 to monitor density fluctuations in pipe flows leading into and out of his tank 10. Each of Smith's control valves 18, 19, and 20 is activated in response to the various readings of the gauges in order to maintain the operation of his device within certain, preset limits.
Nuclear density gauges have also been used with cyclone separators in which the carrier fluid is either a gas or a liquid. Cyclone separators in which the carrier liquid is water are commonly called hydrocyclones and examples of hydrocyclones are: U.S. Pat. No. 3,243,043 to Thompson et al. issued on Mar. 29, 1966, U.S. Pat. No. 3,912,579 to Braun issued on Oct. 14, 1975, U.S. Pat. No. 3,928,186 issued to Zemanek on Dec. 23, 1975, and U.S. Pat. No. 3,334,516 issued to Cedrone on Aug. 8, 1967. In these patents, a flow of particulate solids in water enters the hydrocyclone tangentially through a inlet near the top and begins moving downwardly along a spiral path toward a lower, first outlet. As the flow spirals downwardly, the heavier, particulate solids are thrown outwardly by centrifugal force. The separated particulate solids continue downwardly and exit the hydrocyclone through the lower, first outlet. As the heavier, particulate solids move outwardly, they displace the water and any lighter, particulate solids inwardly toward the middle of the hydrocyclone. The displaced water and lighter, particulate solids exit from the hydrocyclone through a second outlet that is positioned near the top of the hydrocyclone.
To date, no known hydrocyclone operation involving a hydrocyclone design with an inlet and at least first and second outlets such as the ones of Braun, Zemanek, Thompson, and Cedrone has been able to use a nuclear density gauge to monitor density fluctuations over a wide range in the flow within the hydrocyclone. The only known use of a nuclear density gauge in such hydrocyclones is as an on-off switch or warning device to detect the absence or presence of an air core or air column at a particular location within the hydrocyclone. In this known use, the beam of radiation is directed across the hydrocyclone to intersect the hydrocyclone's axis of symmetry between the inlet and the lower, first outlet. The existence or absence of the air core or air column along the axis of symmetry as detected by the nuclear density gauge indicates whether or not the hydrocyclone is operating properly. The existence of this air column in hydrocyclones of the design discussed above is believed to be a result of the spiralling or cyclonic action of the flow within the hydrocyclone and air entrained in the flow or aspirated into the hydrocyclone through the lower, first outlet.
The only known hydrocyclone operation in which a nuclear density gauge is directed across the hydrocyclone and is used to determine the mass of particulate solids being conveyed in the water is U.S. Pat. No. 4,010,369 to Daellenbach et al issued on Mar. 1, 1977. The hydrocyclone of Daellenbach is different both in structure and in operation from the much more common design of hydrocyclones as illustrated by the above-identified patents to Braun, Thompson, Zemanek and Cedrone and as illustrated by the hydrocyclone of the present invention. Daellenbach has a series of hydrocyclones which are inverted and which do not have the customary discharge orifice at the apex end. As shown in his FIG. 4, gamma radiation is directed diametrically across the apex end of the inverted hydrocyclones. The hydrocyclones are connected in cascade as shown in his FIG. 3 so that the overflow from the bottom of one hydrocyclone is supplied as feed to the next hydrocyclone. The purpose of Daellenbach's system is to make an analysis of particle size and mass distribution of the particulate solids in the water. The particle size of each succeeding hydrocyclone is smaller and the variously sized particles are separated out in the apex ends of the cyclones. By measuring across each apex, Daellenbach can then determine the mass of the separated particles in that apex. Daellenbach's hydrocyclones have a single inlet and a single outlet unlike the more common design in which the downwardly tapering cone of the hydrocyclone has an inlet near the top, a first lower outlet at the apex of the cone, and a higher, second outlet near the top of the cone. The hydrocyclones of Daellenbach do not have the problem with air cores or air columns that are present in the more common design and use of hydrocyclones as illustrated by the patents to Braun, Thompson, Zemanek and Cedrone and as employed in the environment of the present invention.
The patents discussed above relate primarily to the use of nuclear density gauges to measure conditions in a flow; however, nuclear density gauges have wide application and can be used, for example, to check the concentricity of tubes as illustrated by U.S. Pat. No. 3,109,095 to Van Horne issued on Oct. 29, 1963.